A polarization multiplexing system has been put to practical use as one of technologies that increase the transmission capacity of an optical signal. In the polarization multiplexing system, a signal may be transmitted by using one pair of polarized waves orthogonal to each other.
A related technology is disclosed in Japanese Laid-open Patent Publication No. 2012-075097 or Non-Patent Document: Zhenning Tao et al., “Multiplier-Free Intrachannel Nonlinearity Compensating Algorithm Operating at Symbol Rate,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 17, Sep. 1, 2011.